Question Issue With System Upgrades

[429CV23]

Having a strange issue with upgrades on my older, AM3+/Phenom II desktop system. I replaced my UPS last week. After a normal shutdown, swapping out the UPS, it would not power up correctly. Power light came on case fans running, no HD activity, no posting, etc. I assumed it was the MB since I had replaced the PSU 6mo ago.
I sourced a Gigabyte B450M-DS3H, Ryzen 5 5600, and 16gb memory. Get it all installed and when powering up, the fans begin to spin for a split second, then nothing, then spin a split second, then nothing.
I assume the PSU is bad, so replaced the Rosewill Hive 650 with a Corsair RM750. No change.
Exchanged the PSU, no change.
I then assumed the MB was bad so exchanged that, same thing.
I tried with the MB sitting on cardboard, no change.
I've gone through disconnecting things until it's just the MB and CPU.
Tried jumping the power switch pins
Put the old MB in, same thing.

So after multiple MBs, PSUs tried, I get the same thing: the case fans spin for a split second, stop, then a split second again, that's it.

I have an ATX PSU tester and a 24pin PSU jumper on the way.

Any ideas?? I've never run into this type issue before.

Thanks for any insight!

 

[rgd1101]

400 series motherboard might need a bios update from r5 5600

 

[429CV23]

Thanks for the reply! Gigabyte claims the newer B450M boards are Ryzen 5 compatible without the BIOS update.

I jumped the green wire in the 24pin harness to ground; the fans power for a fraction of a second then nothing; assuming now I have received another bad psu. Will wait to exchange it until I receive the psu tester tomorrow; will have that with me so the next psu can be tested before I leave the store.